Starting Torque in Motors

Starting torque is the torque produced by a motor when it starts up and begins to accelerate. It is an important parameter to consider when selecting a motor for applications that require high starting torques, such as in heavy-duty machinery or equipment. Among the given options, the capacitor-start motor has the largest starting torque. Let's understand why.

1. Shaded-Pole Motor:
- Shaded-pole motors are single-phase induction motors.
- They have low starting torque, making them suitable for low-load applications like small fans, blowers, and refrigerators.
- The starting torque is generated by the interaction between the shaded pole and the stator windings, resulting in a rotating magnetic field.

2. Repulsion Motor:
- Repulsion motors are also single-phase induction motors.
- They have higher starting torque compared to shaded-pole motors.
- The starting torque is produced by the repulsion between the stator and rotor windings.
- As the motor accelerates, it operates as an induction motor with reduced torque.
- Repulsion motors are used in applications requiring moderate starting torque, such as compressors, pumps, and machine tools.

3. Split-Phase Motor:
- Split-phase motors are commonly used in household appliances and small machinery.
- They have moderate starting torque.
- The starting torque is produced by the phase difference between the main winding and the auxiliary winding.
- The auxiliary winding creates a magnetic field that lags behind the main winding, generating a rotating magnetic field.
- Split-phase motors are suitable for applications with low to moderate starting torque requirements.

4. Capacitor-Start Motor:
- Capacitor-start motors are also single-phase induction motors.
- They have the highest starting torque among the given options.
- The starting torque is produced by a capacitor in series with the start winding.
- The capacitor creates a phase shift, providing a higher starting torque.
- Once the motor reaches a certain speed, a centrifugal switch disconnects the start winding, allowing the motor to operate with reduced torque.
- Capacitor-start motors are commonly used in applications that require high starting torque, such as air compressors, pumps, and conveyors.

Conclusion:
Among the given options, the capacitor-start motor has the largest starting torque. This is because it utilizes a capacitor to provide a phase shift, resulting in a higher starting torque compared to shaded-pole, repulsion, and split-phase motors.